1. Field of the Invention
The present invention relates to an electrochemical element and a process of manufacturing the same. More particularly, the invention is concerened with an improved construction of an electrochemical element using a solid electrolyte to determine the concentration of a component of a gas, which construction is effective to minimize a variation in quality from one element to another. The invention is also concerned with a process suitable for producing an electrochemical element of the above type having such an improved construction.
2. Discussion of the Related Art
There has been known an electrochemical device with an electrochemical element using a solid electrolyte, for example used as an A/F ratio sensor for determining the oxygen concentration of an exhaust gas emitted by an internal combustion engine of an automotive vehicle, or determining the concentration of unburned components of the exhaust gas. An example of this type of electrochemical sensor includes an electrochemical cell with an oxygen-ion conductive solid electrolyte of zirconia ceramics and a pair of porous electrodes, capable of performing an electrochemical pumping action due to the electrode reaction with an electric current flowing between the two electrodes. One of the two electrodes is exposed to an external gas to be measured, through a suitable diffusion-resistance means such as a thin flat space or a porous ceramic layer which has a predetermined diffusion resistance to the molecules of the external gas. A pumping current flowing between the two electrodes is used as an output of the sensor which represents the oxygen concentration of the external gas to be measured, or the concentration of the unburned components of the gas. Also known is an electrochemical detector or device similar to the A/F ratio sensor described above, which is operated to deal with water or detect hydrogen, carbon dioxide, etc., according to the principle of an electrochemical pumping action and a diffusion resistance to the molecules of a component of the gas to be measured.
A typical example of the electrochemical element discussed above is disclosed in U.S. Pat. No. 4,450,065, wherein an electrode disposed on a planar zirconia solid electrolyte layer is exposed to a gas in a thin flat space or gap which has a predetermined diffusion resistance and which communicates with an external space in which the gas exists. The atmosphere adjacent to the exposed surface of the electrode is controlled such that the diffusion resistance of the thin flat space limits an amount of diffusion of the gas through the thin flat space to the exposed surface of the electrode. This electrode cooperates with another electrode to effect an electrochemical pumping action. Further, the electrode exposed to the thin flat space cooperates with yet another electrode also exposed to the thin flat space, in order to achieve an electrochemical sensing operation for sensing an electric current applied across the two pumping electrodes, and thereby detecting the concentration of a component of the gas.
In the electrochemical element with such an arrangement as indicated above the diffusion resistance of the thin flat space communicating with the external measurement gas space is generally determined by a construction or configuration and dimensions of the thin flat space, in particular by the gap amount or thickness of the thin flat space. Attempts have been made to form such a thin flat space so that its thickness or gap is substantially constant over its entire area from the inlet of the gas to its extremities remote from the inlet.
According to the investigation by the present inventors, however, the diffusion resistance of the thin flat space or gap in communication with an external measurement gas space tends to have a considerably large variation from one electrochemical element to another, where the element is produced in a large lot. In other words, the present inventors found it difficult to produce the electrochemical elements with substantially the same diffusion resistance, i.e., with consistent operating characteristics. Thus, a conventional process of manufacturing an electrochemical element suffers difficulty in obtaining a desired value of diffusion resistance of the thin flat space. This difficulty arises from a tendency that solid electrolyte layers, electrode protective layers or other members which define a thin flat space will warp or sag at their portions defining the inlet or open end of the flat space, during a firing process, due to different shrinkage factors of the individual components of the electrochemical element. Such deformation of the members defining the thin flat space results in significant inconsistency of the configuration of the thin flat space from one element to another.
For the production on a commercially justifiable basis of consistently high quality electrochemical elements of the type having a thin flat space communicating with an external measurement gas space, it is important to minimize a variation in the diffusion resistance of their thin flat space from one element to another. Up to the present, however, no efforts have been made to form the thin flat space with a predetermined constant thickness, and thereby minimize a variation in their diffusion resistance.